Method for producing a sheet of paper comprising calcite

ABSTRACT

The invention relates to a method for producing a sheet of paper ( 6 ) comprising paper fibres and calcium carbonate mainly in the form of calcite crystals linked directly to the paper fibres. The inventive method is characterised in that it comprises the following steps:—(i) an aqueous composition comprising calcium bicarbonates ( 5 ) and/or hydrated and/or dissolved carbon dioxide ( 3 ) and (ii) an aqueous composition comprising calcium hydroxide ( 4 ) are mixed in an aqueous medium in such a way as to precipitate the calcium carbonate in the form of vaterite crystals;—paper fibres ( 2, 2 ′) are immediately added;—the vaterite crystals are left to transform into calcite crystals on contact with the fibres;—said mixture containing the calcite crystals which are fixed to the fibres is subsequently sent on the conveying wire of the papermaking machine (M) for drainage and so that the sheet of paper can be formed;—the sheet of paper ( 6 ) thus obtained is treated, if necessary, and dried.

[0001] The present invention concerns a process for the manufacture of asheet of paper containing calcium carbonate in the form of calcite.

[0002] Various types of paper are known to contain mineral fillers, inorder, on one hand, to reduce their costs, since the fillers are lessexpensive than cellulose fibers, and, on the other hand, to provide orimprove certain physical and mechanical characteristics. Materialsspecifically used as fillers are kaolin, talc, titanium oxide, aluminumhydroxide, satin white, and calcium carbonate in ground or precipitatedform. The classic method for these fillers involves preparation ex situat the time of manufacture of the paper; they are incorporated into thepaper fibers and retained through the addition of retention agents.

[0003] The development of paper manufacturing in an alkalineenvironment, in order to make the paper more durable, has led to wideruse of calcium carbonate.

[0004] Calcium carbonate presents a number of crystallographic forms.The most stable and most commonly-used form is calcite; another,somewhat less frequently used form is aragonite; the least stable formis vaterite. Calcite and aragonite crystals are rhombohedral in shape,whereas vaterite crystals are spherical.

[0005] Paper-making processes have been proposed which enable thecalcium carbonate to be precipitated in situ onto paper fibers and fixedwithout the addition of retention agents.

[0006] Such processes have been described in the patents listed below.

[0007] Patent application WO 9942657 describes a new process for thesynthesis of calcium carbonate in the presence of cellulose fibers, soas to obtain, at the end of the process, a calcium carbonate precipitatein situ on the fibers. This process is characterized by the inclusionof:

[0008] a stage involving the realization of a first aqueous compositioncontaining calcium bicarbonate (also known as calcium hydrogencarbonate), which can be obtained by processing calcium carbonate withcarbon dioxide;

[0009] a stage involving the realization of a second aqueous compositioncontaining calcium hydroxide; and

[0010] a final stage in which the first and second compositions aremixed with cellulose fibers, in such a way as to cause the precipitationof calcium carbonate upon contact with at least some fibers.

[0011] When all of these three ingredients are present, the calciumcarbonate crystals are fixed onto the fibers, and a higher level ofretention is obtained than when calcium carbonate is added to a papercomposition that requires retention agents. This process is implementedin the dilute medium typically used in the paper-making process.

[0012] U.S. Pat. No. 5,679,220 also describes a method of fixingprecipitated calcium carbonate onto paper fibers in a paper-makingprocess using cellulose fibers, calcium hydroxide and carbon dioxideunder high shearing as a precipitating gas. The patent envisages avariation in the molar ratio of the carbon dioxide to the calciumhydroxide in order to obtain calcium carbonate crystals of differentmorphologies. Nonetheless, this patent does not clearly show how toobtain the desired crystallographic form of calcium carbonate.

[0013] The objective of the present invention is to provide a sheet ofpaper that contains calcium carbonate in its common form of calcite andthat has improved “look-through.”

[0014] The applicant has shown that it is possible to act oncrystallographic forms of calcium carbonate through the use of the insitu precipitation process of calcium carbonate, by adjusting the orderand the durations of contact between the various compositions involved,in order to precipitate and fix the calcium carbonate onto the fibers insitu when a sheet of paper is being manufactured. She has shown that thecalcium carbonate precipitated according to in situ processes initiallycrystallizes in the form of vaterite, which is unstable before beingtransformed into its more stable form of calcite, and that this passagefrom vaterite to calcite could be controlled and used at the time ofmanufacture of the sheet of paper, in order to improve the fixation anddistribution of the calcium carbonate crystals, in calcite form, withinthe paper.

[0015] The invention provides a process for the manufacture of a sheetof paper containing paper fibers and calcium carbonate, mostly in theform of calcite crystals directly linked to the paper fibers, whereinsaid process includes the following stages:

[0016] mixing, in an aqueous medium, of:

[0017] an aqueous composition containing calcium hydrogen carbonatesand/or hydrated and/or dissolved carbon dioxide,

[0018] an aqueous composition containing calcium hydroxide,

[0019] so as to precipitate calcium carbonate in the form of vateritecrystals,

[0020] immediately adding paper fibers,

[0021] allowing the vaterite crystals to be transformed into calcitecrystals upon contact with the fibers,

[0022] placing this mixture containing the calcite crystals fixed to thefibers onto the wire of the paper-making machine for drainage andformation of the sheet of paper,

[0023] processing (if necessary) and drying of the sheet of paper thusobtained.

[0024] Preferably, the paper-manufacturing process is characterized bythe fact that the composition containing calcium hydroxide is addedafter the composition containing calcium hydrogen carbonates and/orhydrated and/or dissolved carbon dioxide.

[0025] Preferably, the paper-manufacturing process is characterized bythe fact that the composition containing calcium hydrogen carbonatesresults from a mixture, in an aqueous medium, of recycled calciumcarbonate and carbon dioxide.

[0026] Preferably, the recycled calcium carbonate comes from recycledpaper products, specifically recycled paper fibers and/or recycled whitewater.

[0027] Recycled white water originates in the water drained from thefibrous suspension on the wire of the paper-making machine. It containsfine elements, specifically fillers, such as calcium carbonate, andpossibly other alkaline or alkaline-earth compounds and cellulose fibers(called “fine” fibers) which are not retained on the wire, these fibersthemselves containing fillers. Like other possible sources of recycledpaper products, there are recycled paper fibers which originate in oldpapers, specifically de-inked and possibly bleached papers, and recycled“casse paper”; these products also contain fillers, such as calciumcarbonate and possibly other alkaline or alkaline-earth compounds.Another source might be de-inking sludge and other types of sludge fromthe paper-making process.

[0028] According to a particular embodiment of the invention, the carbondioxide is added in different places in the cycle of the paper-makingmachine. Preferably, this addition is performed before the addition ofthe calcium hydroxide, in order to ensure the dissolution of the gas andthus to subsequently encourage the speed of the reaction with thecalcium hydroxide in order to form the precipitated vaterite crystals.

[0029] Preferably, most of the carbon dioxide is added in the course ofthe white water recycling cycle of the paper-making machine. At thestart of the cycle, the carbonic (carbon dioxide) gas participates inthe solubilization of the recycled calcium carbonate, in the form ofcalcium hydrogen carbonates, present in the white water. The calciumcarbonate will subsequently be recrystallized during reaction with thecalcium hydroxide.

[0030] According to a particular embodiment of the invention, theprocess is characterized by the fact that the addition of said gaseouscarbon dioxide takes place between the white water receiving site underthe wire of the paper-making machine and the mineralizer. This willencourage full dissolution of the gas before the reaction with calciumhydroxide. In fact, it is preferable for the carbon dioxide to be addedduring the recycling cycle of the white water, so that it is entirelydissolved, in free form, in hydrated form, or in the form of calciumhydrogen carbonates or other alkaline or alkaline-earth salts, dependingupon which of these ions are present.

[0031] An advantage of this process is that the fibers are not placed incontact with an alkaline product; this improves the homogeneity of thepaper, which is known as “look-through,” because the fibers have atendency to clump together in an alkaline environment. Duringrecrystallization of the carbonate in the presence of fibers, the mediumremains neutral; this improves the efficacy of the sizing agents and theoptical agents, which are alkaline pH-sensitive.

[0032] An advantage of this process, introducing the carbonic gas at thebeginning of the cycle (optimal dilution point for hydration of the gas)and in a machine adapted for mineralization of water, is that it iseasier to use a gas with a low CO₂ content, recovered from a boiler or alime kiln, in order to transform it into calcium hydrogen carbonate.

[0033] According to a particular embodiment of the invention, theprocess is characterized by the fact that the carbon dioxide isintroduced in the form of diluted carbonic gas, specifically fumes froma boiler or a limekiln, containing 8% to 25% of CO₂.

[0034] In the process according to the invention, the molar ratio ofcarbon dioxide to calcium hydroxide is equal to approximately 1, and istherefore stoichiometric.

[0035] Furthermore, the process according to the invention is alsoadvantageously characterized by the fact that the dilution rate of thepaper fibers in the final reaction mixture ranges from 0.1% to 5% byweight, preferably from 0.2% to 1.5%.

[0036] Preferably, the process according to the invention ischaracterized by the fact that the composition containing the calciumhydroxide is an aqueous suspension of solid particles of said calciumhydroxide, known as milk of lime.

[0037] More particularly, the process according to the invention ischaracterized by the fact that the calcium hydroxide is in the form ofan aqueous suspension of solid particles with a grading lower than 10μm, preferably between 0.5 and 2 μm, and particularly on the order of 1μm.

[0038] FIGS. 1 to 3 provide a diagram description of particularembodiments of this process, wherein the relative proportions are notshown in real scale.

[0039]FIG. 1 describes a process according to which a composition ofcalcium hydrogen carbonates (5) is introduced into an aqueous medium, towhich a calcium hydroxide suspension (4) is added in order toprecipitate the vaterite crystals, and, immediately afterward, virginpaper fibers (2) and/or recycled paper fibers (2′) are added and thevaterite crystals are allowed to transform into calcite crystals uponcontact with the fibers, after which the sheet (6) is shaped and drainedon the paper-making machine (M).

[0040]FIG. 2 represents a process according to which carbonic gas (3) isintroduced into the white water recycling cycle (1), after which acalcium hydroxide suspension (4) is added in order to precipitate thevaterite crystals, and, immediately afterward, virgin paper fibers (2)and/or recycled paper fibers (2′) are added and the vaterite crystalsare allowed to transform into calcite crystals upon contact with thefibers, after which the sheet (6) is shaped and drained on thepaper-making machine (M).

[0041]FIG. 3 represents the simplified diagram of a detailed,non-limitative example that describes the manufacture of a paper (6)with a 26% filler of calcium carbonate, principally in the form ofcalcite, in the following manner:

[0042] Let us assume that an industrial installation manufactures 5.4metric tons of paper per hour, characterized by the following operatingconditions for the principal cycles:

[0043] Average flow rate of the suspension into the headbox: 1100 m³ perhour

[0044] Average flow rate of the short recycled water cycle: 1000 m³ perhour

[0045] Average flow rate of the input cycle of virgin fibers andrecycled paper: 100 m³ per hour

[0046] Water temperature in the cycle: 40° C.

[0047] The average composition of the suspension of virgin fibers (2)and recycled paper (2′) consists of 28 g/L of virgin fibers and 16 g/Lof recycled paper coming from the same production, which itself includes12 g/L of fiber and 4 g/L of calcium carbonate (recycled). The averagecomposition of the recycled white water (1) consists of 1 g/L of fiberand 1 g/L of calcium carbonate (recycled).

[0048] Upstream of the pump (P) for recovering the white water (1)recovered under the wire of the paper-making machine, a quantity ofcarbonic gas (3) equal to 440 kg/hour of CO₂ is initially introducedinto the closed short cycle. Then, after the mineralizer (R), milk oflime (4), containing 100 g/L of very fine-grained (approximately 1 μm)calcium hydroxide is added at a flow rate of 7.4 m³ per hour. Finally,the suspension containing virgin fibers (2) and recycled paper (2′) isintroduced.

[0049] The gas reacts almost instantaneously with the milk of lime, inorder to form, in suspension in the water, unstable vaterite crystalsbetween points a and b; subsequently, after being mixed with the fibroussuspension (2, 2′) and before reaching the headbox, the vateritecrystals are transformed into stable calcite. The suspension of fibersand fillers is then conveyed onto the wire of the paper-making machine(M) for drainage and formation of the sheet of paper.

[0050] When the virgin fibers and the recycled paper are added, the pHof the suspension is stabilized at its final value, adjustable between 7and 8 as desired, which corresponds to a calcium-carbon balance of thewater.

[0051] The output of the paper-making machine consists of 5.4 tons ofpaper with a 26% filler of crystals, essentially in the form ofrhombohedral calcite.

[0052] The size of the calcite crystals becomes smaller as the gradingof the milk of lime becomes finer. The paper obtained will accordinglybe more opaque when the milk of lime used contains grains less than 1 μmin dimension.

[0053] The high opacity of the sheet (6) is obtained by virtue of thevery consistent distribution of the crystals hooked on to the fibers,without the help of a retention agent and without any variation in thepH. In comparison with the traditional process of filler retention withformation of aggregates by means of a retention agent, the improvementin opacity is between 3 and 4 points.

[0054] Example From the Prior Art:

[0055] A sheet of paper is made according to the prior art wherebycalcium carbonate is precipitated in situ, mixing the compounds at thesame time without specifically adding the paper fibers last, and withoutadding them immediately after implementing the precipitation of thecalcium carbonate.

[0056] It will be noted that the sheet of paper obtained according tothe invention contains calcite crystals that are better distributed thanthose in the sheet of paper prepared according to the prior art, andthat, accordingly, the “look-through” of said sheet of paper obtainedaccording to the invention is improved.

1. Process for the manufacture of a sheet of paper containing paperfibers and calcium carbonate, mostly in the form of calcite crystalsdirectly linked to the paper fibers, wherein said process includes thefollowing stages: mixing, in an aqueous medium, of: an aqueouscomposition containing calcium hydrogen carbonates and/or hydratedand/or dissolved carbon dioxide, an aqueous composition containingcalcium hydroxide, so as to precipitate calcium carbonate in the form ofvaterite crystals, immediately adding paper fibers, allowing thevaterite crystals to be transformed into calcite crystals upon contactwith the fibers, placing this mixture containing the calcite crystalsfixed to the fibers onto the wire of the paper-making machine fordrainage and formation of the sheet of paper, processing (if necessary)and drying of the sheet of paper thus obtained.
 2. Process according toclaim 1, wherein the composition containing calcium hydroxide is addedafter the composition containing calcium hydrogen carbonates and/orhydrated and/or dissolved carbon dioxide.
 3. Process according to eitherof claims 1 or 2, wherein the composition containing calcium hydrogencarbonates results from a mixture of recycled calcium carbonate andcarbon dioxide.
 4. Process according to claim 3, wherein the recycledcalcium carbonate comes from recycled paper products, specificallyrecycled paper fibers and/or recycled white water.
 5. Process accordingto one of claims 1 to 4, wherein the carbon dioxide is added in variousplaces in the cycle of the paper-making machine.
 6. Process according toone of claims 1 to 5, wherein most of the carbon dioxide is added in thecourse of the white water recycling cycle of the paper-making machine.7. Process according to claim 6, wherein the addition of the carbondioxide takes place between the white water receiving site under thewire of the paper-making machine and the mineralizer.
 8. Processaccording to one of claims 1 to 7, wherein the carbon dioxide isintroduced in the form of diluted carbonic gas, specifically fumes froma boiler or a lime kiln, containing 8% to 25% of CO₂.
 9. Processaccording to one of claims 1 to 8, wherein the dilution rate of thepaper fibers in the final reaction mixture ranges from 0.1% to 5% byweight, preferably from 0.2% to 1.5%.
 10. Process according to one ofclaims 1 to 9, wherein the composition containing the calcium hydroxideis an aqueous suspension of solid particles of said calcium hydroxide,known as milk of lime.
 11. Process according to claim 10, wherein thecalcium hydroxide is in the form of solid particles with an averagegrading lower than 10 μm, preferably between 0.5 and 2 μm, andparticularly on the order of 1 μm.
 12. Process according to one ofclaims 1 to 11, wherein the sheet of paper obtained includes at least10%, and preferably at least 20%, of calcium carbonate of the calcitetype thus precipitated.